PARTHENOGENESIS I4I 



witz;i the J exhibit great variety : usually they are of extremely 

 elongate form, thread-like, with curious sagittate or simply 

 pointed heads, and are of a fibrillar structure, breaking up at 

 various parts into finer threads. 



External Sexual Organs. The terminal segments of the 

 body are usually very highly modified in connexion with the 

 external sexual organs, and this modification occurs in such a 

 great variety of forms as to render it impossible to give any general 

 account thereof, or of the organs themselves. Some of these 

 segments or parts of the segments, for it may be dorsal plates 

 or ventral plates, or both may be withdrawn into the interior^ 

 and changed in shape, or may be doubled over, so that the 

 true termination of the body may be concealed. The com- 

 parative anatomy of all these parts is especially complex in 

 the males, and has been as yet but little elucidated, and as the 

 various terms made use of by descriptive entomologists are of 

 an unsatisfactory nature we may be excused from enumerat- 

 ing them. We may, however, mention that when a terminal 

 chamber is found, with which both the alimentary canal and the 

 sexual organs are connected, it is called a cloaca, as in other 

 animals. 



Parthenogenesis. 



There are undoubted cases in Insects of the occurrence of 

 parthenogenesis, that is, the production of young by a female 

 without concurrence of a male. This phenomenon is usually 

 limited to a small number of generations, as in the case of the 

 Aphididae, or even to a single generation, as occurs in the alterna- 

 tion of generations of many Cynipidae, a parthenogenetic alter- 

 nating with a sexual generation. There are, however, a few 

 species of Insects of which no male is known (in Tenthredinidae, 

 Cynipidae, Coccidae), and these must be looked on as perpetually 

 parthenogenetic. It is a curious fact that the result of partheno- 

 genesis in some species is the production of only one sex, which 

 in some Insects is female, in others male ; the phenomenon in the 

 former case is called by Taschenberg " Thelyotoky, in the latter 

 case Arrhenotoky ; Deuterotoky being applied to the cases in 

 which two sexes are produced. In some forms of partheno- 

 1 Zeitschr. wiss. ZooL 1. 1890, p. 317. ^ Ahh. Ges. ffaUe.xvil 1892, p. 365. 



